Biologists were puzzled by the turtle's behaviour because, despite using its lungs to breathe air, it often submerges its head in water.

By testing the water, they found that the reptile was excreting urea through its mouth instead of its kidneys.

The discovery adds to previous research, which suggested the turtles have highly specialised mouth tissues.

Professor Ip Yeung Kwon and colleagues from the National University of Singapore published their study in the Journal of Experimental Biology.

The species, Pelodiscus sinensis, is found in swampy, brackish water and is native to much of East Asia.

Their unusual mouths were first discussed over a century ago when researchers suggested that the velvety tissues inside function in a similar way to gills in fish.

Biologists theorised that the mouth tissue may play a role in oxygen and salt filtering but according to Prof Ip the processes were not "clearly defined".

On the whole, the turtles respire in the same way as most other members of the chelonian family.

Whether basking in the sun to warm their blood or "snorkelling" at the surface of the water with their snouts in the air, the turtles breathe by pulling air into their lungs.

But observations of the species noted that they occasionally submerge their heads into pools of water for up to 100 minutes.

Prof Ip and colleagues brought a turtle into the lab to study how the turtles did not drown and to observe what else may be happening.

Researchers provided the turtle with water and observed as it regularly dipped its head and rinsed water through its mouth.

The turtle's rhythmic motion of its throat, not to mention its survival, signalled that it was indeed breathing during these submerged spells.

Scientists also analysed how the chemistry of the water changed after the turtle interacted with it and found increased levels of the chemical compound urea.

The majority of vertebrates expel urea, a waste product full of nitrogen, through urine via the kidneys.

In turtles urea passes out of the cloaca - the single orifice used for waste matter and reproduction.

"Throughout [the study] period, the urea excretion rate through the mouth was significantly greater, 15- to 49-fold, than that through the cloaca," said Prof Ip.

"These results indicate for the first time that [mouth tissue] processes and rhythmic [throat] movements were involved in urea excretion in P. sinensis."

Prof Ip told BBC Nature that the ability to pass waste through the mouth was unique to this species. The turtles' leathery shells are unlike the hard carapaces of other turtles But he suggested that the ability could be evolutionarily linked to how some mammals such as bats, cattle and goats "recycle" nitrogen by excreting urea through their saliva.

"We were greatly surprised by our novel results because it is generally accepted that the kidney is responsible for the excretion of urea in vertebrates - except fish," he said.

"Contrary to this common notion, they suggest that the mouth can be a major route of urea excretion in soft-shelled turtles."

Chinese soft-shelled turtles are a delicacy in many parts of Asia and are farmed extensively. A recent survey of 684 farms in China suggested they sold 91 million turtles every year.

However, wild populations are listed as "Vulnerable" by the International Union for the Conservation of Nature due to the pressures associated with this farming.